The invention relates to a tensioning device for expanding or stretching a threaded bolt as a result of tension on the threaded portion thereof. The tensioning device comprises                a support tube which surrounds the threaded bolt and which is supported against a base which surrounds the threaded bolt as an abutment,        a cylinder housing which is arranged in the extension of the support tube having at least one piston which is movable therein in the longitudinal direction and delimits a hydraulic operating chamber together with an inner wall of the cylinder housing,        an interchangeable bush which is rotatable in relation to the piston, is realized so as to be able to be entrained axially by the piston and is provided with an internal thread for screw-connection to the threaded portion of the threaded bolt,        and a device for path and/or position detection which is comprised of two interacting elements.        
A hydraulically operating threaded bolt tensioning device with said features is known from WO 2013/117807 A1. It serves for tightening screw connections or threaded bolts which are subject to high stresses. Using hydraulic pressure, a high tensile force can be applied temporarily onto the threaded bolt in order to tighten or re-tighten at the same time the nut fitted on the threaded bolt in a torsion-free manner. To this end, an interchangeable bush is screwed onto the threaded portion of the threaded bolt which projects beyond the nut. The interchangeable bush communicates axially with a hydraulically driven piston. When hydraulic pressure is introduced, the piston is raised and entrains the interchangeable bush, as a result of which the threaded bolt is pulled a little in the longitudinal direction and at the same time is also stretched. In order to gain information during the tensioning process concerning the movement of the interchangeable bush and consequently of the bolt end screwed into the interchangeable bush, the interchangeable bush is provided with a magnetic strip in the region of its threaded engagement into the threaded bolt. An electric sensor, by way of which the longitudinal movement of the magnetic strip is able to be measured, is arranged at approximately the same height in the cylinder housing of the tensioning device.
Good measurement results can be achieved in most cases with the known tensioning device. However, false results can arise in situations in which light torques act on the cylinder housing. This is typically the case when the device is not used for perpendicular screw connections but at an angle or even in a horizontal tensioning direction. For asymmetries at the housing or the periphery, for instance the hydraulic supply connected on one side of the cylinder housing, can lead to the cylinder housing rotating a little about the tensioning axis with a decrease in the hydraulic pressure, and in this case also resulting in the interchangeable bush which is screwed onto the bolt end twisting slightly. Said interchangeable bush or the magnetic strip fastened thereon is no longer able to supply the sensor with a reliable position.
Consequently, it is the object of the invention to obtain reliable measurement results even in such particular situations.